3 Programming. Jalal Kawash Mandatory: Chapter 5 Section 5.5 Jalal s resources: How to movies and example programs available at:

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1 3 Programming 1 Mandatory: Chapter 5 Section 5.5 Jalal s resources: How to movies and example programs available at: JT s resources: Reading Assignment 2 1

2 To simulate the possibility of different events occurring as a program is a run a random number function can be employed. Typically the function generates a number within a certain range: 1 2 (coin flip) 1 6 (normal die roll) (percentage) Etc. JT s Extra: Random Numbers Example 9: random numbers It can be invoked by the random function of class World The only values generate: 1, 2, 3, 4, 5, 6 (no real numbers) Inclusive Exclusive JT s Extra: Dice Example 2

3 If-Else Statements Alice makes decisions 5 At the end of this section, you will be able to: 1. Understand conditional execution 2. Understand and use the Alice if/else statement 3. Use functions as arguments to other methods 4. Use conditional structures in flowcharts Objectives 3

4 Frequently this is needed when the program must react to user input or the actions of the user. New: series of instructions Open: series of instructions Save: series of instructions Etc. JT s Extra: Choosing Among Alternative Course Of Action It s the mechanism used in many programming languages used to execute different instructions when alternatives are faced. At the branch (where the decision must be made) a logical statement determines which alternative occurs. Another way of looking at it: a question is asked which has a true or false answer. The answer to the question determines which branch executes. The flowchart symbol used to represent a branch/decision is a diamond. Question Branching/Decision Making 4

5 Branching/Decision Making (2) Programs need to make decisions An ATM program must verify your password to know how to proceed This is called conditional Execution The execution of a group of statements is conditional on a Boolean condition If the password is correct then go to the next step Else give an error message Conditional Execution JT (new terminology): body is the part that executes only under a certain condition 5

6 Statement before if true Test if condition false if body else body If-Else Flowchart Statement after if 11 JT: if body JT: else body If-Else Statement in Alice 12 6

7 Example Story Line 11/19/ Returns a random number between 0.1 and 4 value substituted for distance parameter of move Use of Functions Returns a Boolean value 14 7

8 When the if condition is true 15 When the if condition is false 16 8

9 If you want the program to react when the logical statement evaluates to true (the answer to the question is true). Penguin falls into the hole Hole wider Than Penguin? (Rest of the program) Just use the if and leave the else portion empty. Indented part after the if : what executes if the statement is true JT, One Action: Do Something When True You want the program to do something when the logical statement evaluates to true and something different when the statement evaluates to false. TRU E Penguin falls into the hole Hole wider Than Penguin? Penguin: Wow! (Rest of the program) Add instructions for both the if and the else portion of the instruction. JT: Do Something For Both True/False Cases 9

10 Are used in conjunction with logical statements and relate or compare things. The comparison will result in a true or false result. E.g., temperature is above freezing (greater than zero). Accessing the logical operators in Alice. Look under the functions of the world object. JT: Logical Operators Desired comparison Equality == Operator (Alice) Example Not equal to (inequality) JT: Types Of Logical Operators!= Greater than > Greater than, equal to >= Less than < Less than, equal to <= 10

11 If a number of questions must by asked by a program and the answer to one question has no impact on the answer to the other questions. That means that all questions will be asked regardless of whether any questions answered true or false. Q1 Do something Q2? Do something Rest of program JT: Multiple Branches: Independent If a number of questions must by asked by a program and the answer to one question has no impact on the answer to the other questions. That means that all questions will be asked regardless of whether any questions answered true or false. Q1 Do something Always ask second question Q2? Do something Rest of program JT: Multiple Branches: Independent 11

12 Example 10: multiple successive branches First Boolean question: Always asked Second Boolean question: Not indented, always asked JT s Extra: Multiple Branches 23 Sometimes the answer to one question will determine whether or not other questions will be asked. The answer to one question (true or false) has an effect on whether another question is asked. The asking of the second question is dependent on the answer to the first question. (Alternatively: the second question is nested inside the first question). Nested Branches 12

13 Q1 Q1 Q2? Do something Q2? Do something Rest of program Nested Branches: Flowchart Rest of program Q1 Answer to first if determines whether second if executes Q1 Q2? Do something Q2? Do something Rest of program Nested Branches: Flowchart Rest of program 13

14 Multiple if/else statements can be used in succession. Example program 11: nested_branches Indented: executes if first if is true JT: Multiple Branches: Dependent (RHS Case) Indented twice: only executes if other cases answer false Also indented: execution is dependent on the first if (Only ask about blue if user does not indicate that red is the favorite) JT: Multiple Branches: Dependent (LHS Case: Nonsensical) 28 14

15 Loops Do it again 29 At the end of this section, you will be able to: 1. Understand repetition structures 2. Use the loop statement 3. Use the while statement 4. Understand when to use a loop or a while 5. Use loop structures in flowcharts Objectives 15

16 A loop statement allows a group of statements to be repeated for a number of times loop for 5 times monkey.monkeyjump monkey.move(up,1 meter) monkey.move(down,1 meter) Loop Statement 31 JT s Extra: Adding A Loop In Alice 16

17 Loop Statement in Alice 33 Start Loop 5 times Monkey Jumps Monkey moves up 1m Monkey moves down 1m Stop Loop in Flowcharts 34 17

18 Loops are used to repeat a part of the program (or potentially the entire program itself). They are similar to branches where a logical statement is employed. In the case of a loop if the logical statement evaluates to true then a part of the program will repeat (loop). The repetition will continue until the logical statement evaluates to false. JT s Extra: Boolean-Controlled Loops Statement before while true Test while condition false while body Statement after while While Statement 36 18

19 Starting value Example 11 Condition for repeating Loop body : Instructions that are repeated JT s Extra: Counting With While 37 START count = 1 count <= 5? True False Troll counts, arm moves up Arm moves down Ground cries out Arm moves up, count = count + 1 END JT s Extra: Example Flowchart 38 19

20 While Example 39 Example 12 If-Else branch is part of the body of the loop JT s Extra: Combining Loops And Branches 40 20

21 While Example 41 21